> Generally, I would agree, but I've seen cases where it mattered, because the value of a feature would become 0 after a set date. For instance, we needed a specific feature to fulfill the requirements for a (massive) contract

As I said, you put it at the top of the backlog and alert the team of a new deadline. You work on it until the deadline. If you make it, great, if not, you move on. Asking for an estimate wouldn't have made you make it, it might've only prevented you from trying.